GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 85 



skeletal muscles, physiologically as well as anatomically. They are stumpy, 

 quadrangular cells, which are not known to have a sarcolemma, and which are 

 united not only by their broad ends, but by lateral branches. Engelmann 

 and others have considered conduction to take place in the heart from cell to 

 cell, without the intervention of nerves, and in all directions with equal readi- 

 ness. This view was held because the irritation was found to spread in all 

 directions through the muscle-substance, and no nerves had been discovered 

 which could account for this free communication. Quite lately, however, 

 Hegmans and Demoor claim to have discovered in the heart of the frog, by 

 the Golgi staining method, an anastomosing network of nerve-fibres which 

 extends over the whole heart. This nervous network would appear to give 

 ample means of communication between the different parts of the heart, 1 but 

 it is possible that it has only a regulatory function. 



The cells of the contractile substance of some of the medusae (as Aurelia), 

 have been supposed to communicate by contiguity rather than by continuity. 

 The same has been thought to be the case with many forms of unstriated 

 muscle-tissue ; 2 moreover, there are groups of ciliated cells, the members of 

 which act in unison although they have not been found to be connected either 

 directly or by nerves. These cells have apparently no membranous covering, 

 and though living as independent units, are so related that a condition of 

 activity excited in one seems to be transmitted to the rest by means of contact, 

 or through the mediation of cement-substance. 



From what has been said it will be seen that protoplasmic continuity 

 ensures free communication between different cells ; that protoplasmic con- 

 tiguity, either directly or through the mediation of the cement-substance, may 

 possibly permit of conduction ; but that the intervention of a different tissue, 

 even as delicate as the sarcolemma, suffices to cause complete isolation of the 

 cell from its neighbors. 



Transmission of Excitation by means of End-organs. The latest 

 researches on the anatomy of the spinal cord seem to show that the incoming fibres 

 do not communicate directly with nerve-cells, but terminate in brush-like end- 

 ings in the immediate vicinity of the cells. A similar arrangement is found 

 wherever nerve-cells are excited to action by nerve-fibres. It is doubtful 

 whether the brush-like endings should be regarded as special exciting mechan- 

 isms, or whether the brush endings should be considered to be in contact with the 

 nerve-cells or their protoplasmic processes, and this relation to be sufficiently 

 close to permit the cells to be stimulated. The former view is favored by the 

 fact that though the end-brush can excite the cell, the cell does not seem to be 

 able to excite the brush. Much the same can be said of the end-plates by 

 which the condition of excitation of nerve-fibres is conveyed to muscle-fibres, 

 for they seem to be in contact with, rather than continuous with the muscle- 

 substance. Though the nerve end-organ can excite the muscle, the muscle 

 does not appear to be able to excite the nerve. 



1 Archives de Biologic, 1895, vol. xiii., No. 4, p. 619. 



2 Engelmann : Pfluger's Archiv, 1871, Bd. iv. 



